This invention relates to an ATM (Asynchronous Transfer Mode) signal switching apparatus and, in particular, to a switching reference in switching ATM virtual paths (VP""s: Virtual Paths) and ATM virtual channels (VC""s: Virtual Channels) in order to obtain high reliability for the ATM network formed by the signal switching apparatus. This invention relates also to a device and a method for transferring failure information as the switching reference in the ATM network.
For convenience of description, VP switching alone will be described hereinafter. VC switching is carried out in a manner similar to the VP switching.
in the prior art, the following methods are known.
(1) VP AIS (Alarm Indication Signal) Based Switching Method
Upon occurrence of a line failure, a path failure (for example, an STS-1 path failure), or a failure in a VP itself, a VP AIS is inserted downstream. At a receiving end, detection of the VP AIS is used as a switching reference (i.e., switching trigger).
The above-mentioned method is described also in T1.X1.5/96-044 Alcatel Contribution xe2x80x9cATM UP Uni-Ring Fault Reconfigurationxe2x80x9d. An AIS based switching method itself has been generally practiced in a SONET (Synchronous Optical Network). In this method, the path failure is transmitted via a Path AIS and the detection thereof is used s the switching trigger. A UPSR (Unidirectional Path Switched Ring) of the SONET adopts this method (GR-1400-CORE Issue 1, Bellcore March 1994).
(2) PM (Performance Monitoring) Based Switching Method
By PM carried out at a VP terminating point, the status of the VP is detected.
It is used as a switching reference (switching trigger). This method is also described in T1.X1.5/96-044 Alcatel Contribution xe2x80x9cATM VP Uni-Ring Fault Reconfigurationxe2x80x9d.
(3) Switching Message Based Switching Method
There is a method of switching in response to notification or delivery of a switching message. For example, in a SDH (Synchronous Digital Hierarchy) network, protection using K1 and K2 is widely used. As an application of the above-mentioned scheme to an ATM, there is a method of delivering the message by the use of OAM (Operation: Administration and Maintenance) cells.
JP-A No. 157942/1992 entitled xe2x80x9cFault Announcing Cell Generating Apparatusxe2x80x9d discloses a method of regenerating fault announcing cells for notification. In T1.X1.5/96-044, T1X1.5/96-087, and T1S1.5/96-087, Alcatel contribution entitled xe2x80x9cATM VP Uni-Ring Fault Reconfigurationxe2x80x9d a method of transmitting VP Segment Status/Link Status messages by the use of regenerated cells is proposed.
JP-A No. 235983/1993 entitled xe2x80x9cVirtual Path Switching Apparatus and Switching Methodxe2x80x9d describes a technique of detecting a fault by path monitoring means, propagating a switching control signal on a spare channel to maintain a capacity, and thereafter performing the switching operation, although a practical propagation scheme is not described in detail.
JP-A No. 287539/1992 entitled xe2x80x9cAlarm Transfer Method and Transfer System in an ATM Networkxe2x80x9d describes an alarm transfer method in which the failure is restricted to a link failure and all VPI""s (Virtual Path Identifiers) accommodated in a link in question are dealt with. The VIP""s are recognized from a VPI management table. For each of the VPI""s, a cell containing alarm information is formed and delivered.
Problems in the switching operation triggered by the VP AIS are:
(1) It is difficult to insert the VP AIS for VP""s up to 4096 at maximum (for VC""s, much greater). In addition, insertion must simultaneously be carried out at a high speed.
(2) This means that new OAM cells are inserted. It is therefore necessary to newly assign the OAM cells to a transmission path band currently used. In those cases where user cells are completely lost, such as Line LOS (Loss of Signal), the insertion of VP AIS as the OAM cells results in no problem. However, in those cases where the user cells are not lost, for example, in the case of B2 errors, the insertion of the VP AIS may suppress the ban allotted to the user cells.
(3) The use of the OAM cells inevitably result in a disadvantage of slow message transmission. The insertion of the VP AIS requires at least 500 msec before the first insertion takes place. Subsequent insertion is carried out at a rate of 1 cell/1 second (GR-1248-CORE Issue 1, Bellcore August 1994).
Accordingly, when the switching operation is performed in response to the detection thereof, several seconds are required taking a VP AIS detection guard time into consideration. Thus, the switching operation is very slow.
Problems in the switching operation triggered by the VP PM are:
(1) Up to 30 VP PM cells are simultaneously monitored at maximum (GR-1248-CORE Issue 1, Bellcore August 1994). Thus, the objects to be switched are restricted in number to 30.
(2) The insertion of the VP PM cells is carried out at a rate of 1 cell/128 cells (depending upon the case, 1 cell/256 cells, 1 cell/512 cell, 1 cell/1024 cells) (GR-1248-CORE Issue 1, Bellcore August 1994). Therefore, switching by this approach requires a long switching time.
Problems in the switching operation in response to the switching message in the OAM cells are:
(1) Communication is required to transmit the message between nodes. This requires insertion of new OAM cells and complicated message processing for those OAM cells, As a result, a processing circuit has a corresponding increase in scale.
(2) Since the new OAM cells are inserted, these OAM cells are newly assigned to a transmission path band currently used. This may suppress the band allotted to the user cells. Generally, the number of bits required in generation of n cells for 4096 VP""s is calculated by nxc3x97 53xc3x978xc3x974096=n=1.7 Mbit.
In case of generation of n cells per one second, the band required is calculated as nxc3x9753xc3x978xc3x974096= nxc3x971.7 Mbps.
(3) Delivery of the switching message requires message recognition and communication. This requires additional processing time which will result in slow switching.
The slow switching is considered as follows.
The insertion of VP PM cells prescribed in GR-1248-CORE is carried out at a rate of 1 cell/128 user cells. For low-frequency errors, small cell loss, and misinsertion, no trouble is caused if the switching operation takes a slightly longer time resulting in no serious influence imposed upon the application. On the other hand, upon occurrence of serious failure such as Line LOS, the switching operation must be performed as soon as possible. Accordingly, a problem arises if the same approach is applied and the switching time is uniformly delayed.
The above-mentioned methods have critical disadvantages such as increased scale of the processing circuit related to the switching operation, longer processing time, suppression of the band allotted to the user cells, and slower switching.
It is therefore an object of this invention to provide a device and a method which is capable of transferring, in an ATM network, failure information as a switching reference at a high speed.
It is another object of this invention to overcome various disadvantages including increase in scale of a processing circuit related to switching, a long processing time, and suppression a band allotted to user cells.
Other objects of this invention will become clear after reading the following description.
A node, to which this invention is applicable, is used in an ATM (asynchronous transfer mode) network and is supplied with user cells. The node includes a failure detecting section for detecting a failure which has occurred in the ATM network such that no user cell is supplied to the node.
According to a first aspect of this invention, the node comprises: band detecting means for detecting, as a detected band, a transmission path band used in transferring the user cells immediately before occurrence of the failure; and producing means connected to the failure detecting means and the band detecting means for producing, when the failure detecting means detects the failure, OAM (Operation: Administration and Maintenance) cells at a production rate increased within a range such that the detected band is not exceeded. The OAM cells are for use in transferring information indicative of occurrence of the failure.
According to a second aspect of this invention, the node comprises: band memorizing means for preliminarily memorizing, as a memorized band, a transmission path band used in transferring the user cells immediately before occurrence of the failure; and producing means connected to the failure detecting means and the band memorizing means for producing, when the failure detecting means detects the failure, OAM (Operation: Administration and Maintenance) cells at a production rate increased within a range such that the memorized band is not exceeded, the OAM cells being for use in transferring information indicative of occurrence of the failure.
A node to which a third aspect of this invention is applicable is for use in an ATM (asynchronous transfer mode) network and is supplied with a succession of monitoring objects which is one of a succession of VP""s (Virtual Paths) and a succession of VC""s (Virtual Channels). The node includes performance monitoring means for carrying out performance monitoring on a predetermined number of the monitoring objects among the succession of monitoring objects.
According to the third aspect of this invention, the node comprises: dividing means for dividing the succession of monitoring objects into a succession of object blocks, each object block comprising the predetermined number of the monitoring objects, the dividing means successively delivering the objects blocks to the performance monitoring means.
A method to which a fourth aspect of this invention is applicable is for transferring information indicative of occurrence of a minor failure which is detected in a node of an ATM (asynchronous transfer mode) network and which is occurred in the ATM network such that user cells are allowed to continuously flow in the ATM network.
According to the fourth aspect of this invention, the method comprises the step of: using, in the node, bits of the user cells in transferring the information so that the information is informed without addition of new cells and without preventing communication of user information of the user cells.
A method to which a fifth aspect of this invention is applicable is for transferring information indicative of occurrence of a minor failure which is detected in a node (14) of an ATM (asynchronous transfer mode) network and which is occurred in the ATM network such that user cells are allowed to continuously flow in the ATM network.
According to the fifth aspect of this invention, the method comprises the step of: producing, in the node, new cells using a different transmission path band which is different from a transmission path band used in transferring the user cells, the new cells being for use in transferring the information.